Hydraulic arrangement for dockboards

ABSTRACT

A hydraulic arrangement for use in dockboard installations which include a hydraulic cylinder for raising the dockboard. The arrangement allows the dockboard to be raised to a position above the vehicle platform, and then to be lowered onto the vehicle platform, all by a single operation of a switch which energizes the hydraulic pump motor. The hydraulic arrangement includes a special control valve situated between the pump and hydraulic cylinder, the valve being also connected to a reservoir, and arranged so that when the pump is operating at normal pressure the control valve allows fluid to flow to the cylinder so raising the dockboard, and when the pump motor is stopped the control valve allows fluid to flow from the cylinder to the reservoir allowing lowering of the dockboard. The circuit includes further valve means to assist reduction of pressure at the pump outlet when the pump motor is stopped, to speed operation of the valve.

1451 Oct. 3, 1972 [54] HYDRAULIC ARRANGEMENT FOR DOCKBOARDS [72] Inventor: Dermont F. Loblick, Gibbons, Al-

berta, Canada [22] Filed; June 26,1970

[21] Appl. No.: 50,256

[30] Foreign Application Priority Data July 4, 1969 Great Britain ..33,904/69 152] Us. c1 ..14/71, 60/52 R, 91/401 [51] 1m. c1. .365g 11/12, F156 15/18 [5s] Field of semh ..60/52 T, 91/401, 14/71 [56] References Cited UNITED STATES PATENTS 1,904,345 4/1933 Anthony et a1. ..91/401 2,061,530 11/1936 w11e ..60/52 T 2,072,481 3/1937 McNairy ..60/52 T 2,082,613 611937 Bower ..60/52 T 2,313,056 3/1943 Emerson et a1. ..60/52 R 2,331,603 10/1943 Falcon ..60/52 Hc x 2,630,272 3/1953 Rack e1 a1. ..60/52 T 3,148,591 9/1964 sheesley ..91/401 FOREIGN PATENTS OR APPLICATIONS 951,124 4/1949 France ..9l/40l Primary Examiner-Edgar W. Geoghegan Attorney-Cushman, Darby & Cushman [57] ABSTRACT A hydraulic arrangement for use in dockboard installations which include a hydraulic cylinder for raising the dockboard. The arrangement allows the dockboard to be raised to a position above the vehicle platform, and then to be lowered onto the vehicle platform, all by a single operation of a switch which energizes the hydraulic pump motor. The hydraulic arrangement includes a special control valve situated between the pump and hydraulic cylinder, the valve being also connected to a reservoir, and arranged so that when the pump is operating at normal pressure the control valve allows fluid to flow to the cylinder so raising the dockboard, and when the pump motor is stopped the control valve allows fluid to flow from the cylinder to the reservoir allowing lowering of the dockboard. The circuit includes further valve means to assist` reduction of pressure at the pump outlet when the pump motor is stopped, to speed operation of the valve.

5 Claims, 8 Drawing Figures PMENTED URI 3 |972 Smau 1 nr 4 HYDRAULIC ARRANGEMENT soa y lDoclrBoAims The present invention relates to a hydraulic circuit arrangement particularly for dockboard installations. Such installations are used to provide a sloping ramp or dockboard connecting the floor of a warehouse bay or like facility to the platform of a vehicle being loaded or unloaded.

Warehouses and like places generally have loading and unloading bays which are raised from ground level so as to be of similar height to load carrying platforms of the vehicles which are serving the bay. ln order for goods to be moved readily from the vehicle platforms on to the loading bay and vice versa, it is necessary to provide a dockboard which spans the gap between the vehicle and the loading bay, and which also provides a ramp between the loading bay and the vehicle where these are not quite level. Various types of dockboard installations have been developed which include a base part to which the dockboard is hinged at its rear end, and which incorporate mechanically or hydraulically operated means for raising the dockboard and lowering this onto the platform of a vehicle.

Various dockboard installations have been designed in which the dockboard is raised and lowered by operation of a hydraulic cylinder or cylinders. The hydraulic cylinder may act either directly between pivot points on the base structure and the dockboard itself, or the dockboard may be raised by a linkage type mechanism which obtains its motive force from a hydraulic cylinder. An example of a linkage type mechanism operated by a hydraulic cylinder is described in my copending Canadian Pat. application No. 075,583, filed Feb. 23, 1970 (corresponding to U.S. Ser. No. 50,255).

The dockboards generally include an extendable lip at their outer edge which rests on the vehicle platform, andA my copending Canadian Pat. application No. 075,584 (corresponding to U.S. Ser. No. 50,098) describes a suitable form of lip operating mechanism for a lip which is hinged to the outer edge of the dockboard and extends when the dockboard is raised, but which allows the lip to fold away flush with the edge of the loading bay when the board is lowered and not in use. ln this condition, the lip may itself support the dockboard.

Although the present invention provides an arrangement particularly suited for use with the hydraulically operated mechanism described in my copending Pat. application No. 075,583, nevertheless the hydraulic circuit described herein may be used with many different types of dockboard installations which utilize a hydraulic piston and cylinder combination effective to raise the dockboard when supplied with pressurized fluid.

The hydraulic arrangement in accordance with the invention allows the dockboard to be raised to a position above the vehicle platform in which position the dockboard lip is extended, and then to be lowered onto the vehicle platform until supported by the lip, all by a single operation of a switch which energizes the hydraulic pump motor. The switch is held closed until the dockboard has reached its raised position, and when the switch is opened to de-energize the pump motor a control valve in the system automatically connects the hydraulic cylinder to a reservoir, so that the dockboard lowers under its own weight until the lip of the dockboard contacts a vehicle platform. After the vehicle has been loaded or unloaded and has been driven away, .the dockboard continues to move downwardly until in its "at rest position. The copending patent applications referred to above disclose means whereby the lip is automatically collapsed when the vehicle moves away or when the dockboard reaches the at rest position.

The basic elements `of the hydraulic circuit in accordance with the invention are:

a. a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with pressurized fluid;

b. a pump for supplying pressurized fluid to said cylinder from a reservoir;

c. a pressure operated control valve connected by conduitmeans to the pump outlet, the cylinder and the reservoir.

The control valve has a valve member movable from a first position, in which it allows flow of fluid from the hydraulic cylinder to the reservoir, to a second position in which it prevents f'low of fluid from the hydraulic cylinder to the reservoir. The valve member is responsive directly to fluid pressure at the pump outlet so as to move to its second position when the pump is delivering fluid at normal working pressure and biassed, for example by spring means, so as to return to its first position when the pressure at the pump outlet is substantially reduced. Thus, when the pump is started the pressure rise at the pump outlet causes the valve member to move to its second position and allows fluid to pass from the pump to the hydraulic cylinder while preventing flow from the cylinder and so raising the dockboard, and when the pump is stopped pressure reduction at the pump outlet allows the valve member to return to its first position, so that fluid may f'low from the cylinder to the reservoir, allowing the dockboard to lower under its own weight.

The system so far described depends for its practical operation on a substantial reduction of pressure occurring at the pump outlet after the pump has stopped with the piston of the hydrauliccylinder at or near the end of its stroke. This pressure reduction is essential to allow for the valve member to return to its first position when the pump is stopped.

Some pressure reduction in the pump outlet when the pump is stopped is probably inevitable due to flow restrictions in the circuit and backwards flow through the pump; however this pressure reduction may occur too slowly for satisfactory operation of the circuit.

The present invention accordingly provides further valve means in the hydraulic cylinder circuit, these valve means being arranged to aid reduction of pressure at the pump outlet when the pump motor is stopped with the piston of the hydraulic cylinder near the end of its stroke, thus facilitating the return of the valve member to its first position to allow lowering of the dockboard to commence.

As explained more fully below, these further valve means may be variously arranged in the hydraulic cylinder circuit, i.e., that part of the hydraulic circuit which includes the conduit leading from the pump to the hydraulic cylinder on the downstream side of the connection between the pump and the control valve, and the part of the hydraulic cylinder or any other part communicating directly with that conduit.

The provision of these further valve means in accordance with the invention ensures that no undue delay occurs between the moment when the pump is stopped with the piston near the end of its stroke, and the moment when the valve member has returned to its first position to allow lowering of the dockboard to commence. Without the provision of some such valve means, the pressure which occurs when the cylinder nears or reaches the end of its stroke (which is higher than the average pressure during the raising of the dockboard) is communicated back to the pump outlet, and the valve member cannot return to its first position until this relatively high pressure has been relieved by flow of fluid back through the pump. In these circumstances the valve member tends to remain in its second j position, and operation of the dockboard is delayed.

The further valve means for aiding reduction in pressure in the first conduit means in accordance with this invention is a relief valve operable to relieve pressure from the hydraulic cylinder circuit to the resevoir, the valve being operated by valve actuating means arranged to operate the valve as the piston nears the end of the stroke. ln a particular preferred arrangement, this valve is movable with the piston of the hydraulic piston and cylinder arrangement and is operated by actuating means at the end of the cylinder. Upon opera- 'tion the valve allows fluid to pass from the end of the cylinder supplied with pressurized fluid to the other end of the cylinder which has an outlet connected to the reservoir. The valve actuating means is preferably adjustable to allow for adjustment of the stroke of the piston.

Another form of valve means which is also of assistance in aiding reduction in pressure in the pump outlet when the pump motor is stopped is a non-return valve in thehydraulic cylinder circuit effective to iso'- late a high pressure developed in the hydraulic cylinder from the pump outlet after the pump motor'is stopped, so that when this stage has occurred it only requires a slight leakage back through the pump to rapidly reduce the pressure at the pump outlet. The non-return valve may be incorporated in the control valve. ln the preferred embodiment of the invention both the relief and the non-return valve are used. The relief valve mounted in the piston acts to reduce pressure rapidly when the piston has completed its stroke, and prevents any build-up in pressure which would otherwise occur in this condition. The non-return valve is principally useful in the circumstances that the pump motor is stopped before the piston has moved far enough to allow the relief valve to be operated; in this case the non-return valve prevents an undue time lag that would otherwise occur before pressure in the pump -outlet is reduced sufficiently to allow the valve member to return to its initial position.

In the preferred hydraulic circuit the control valve is interposed between the pump outlet and the hydraulic cylinder, so as to control flow of fluid from the pump to the cylinder as well as from the cylinder to the reservoir. ln this circuit there are provided first conduit means connecting the pump to the valve, second conduit means connecting the valve to the cylinder, and

third conduit means connecting the valve to the reservoir.

The valve member is movable from a first position in which it prevents communication between the firstand second conduit means and allows communication between the second and third conduit means, to a second position in which it allows communication between the first and second conduit means and prevents communication between the second and third conduit means. The valve member is urged towards the first position by 'spring means, which are preferably adjustable, and is responsive to fluid pressure in the first conduit means so as to move to the second position when the pump is supplying fluid to the first conduit means at normal working pressure.

The valve preferably includes an inlet communicating with the first conduit means, and main outlet communicating with the second conduit means, and the valve member is made responsive to pressure at this inlet to move to its position where it allows for direct communication between the inlet and the main outlet. This is in contrast to known shuttle valves operable by fluid pressure but which require separate connections from the pump outlet to the pressure sensing portion of the valve and to the inlet of the valve.

The valve conveniently has a bore in which the valve member is slidable in sealing relationship with the walls of the bore. One end of the bore communicates with the first conduit means, and the main outlet of the valve communicates with a first port in the bore which is positioned to be obturated by the valve member when in its first position, but uncovered when the valve member is in its second position. The valve bore includes a second inlet port communicating with the second conduit means, which second port is arranged to be obturated by the valve member in its second position but uncovered when the valve member is in its first position, allowing fluid to flow into the valve through the second conduit means and out through the third conduit means.

This control valve may also include a pressure relief feature in the form of a third port connected to the reservoir and so positioned in the valve bore as to be obturated by the valve member in both its first and second positions, but uncovered by the valve member when this moves beyond its second position by reason of excess pressure at the inlet, in which case direct communication is provided between the inlet and the reservoir.

A further very useful feature'of the control valve in accordance with this invention is that the spring means which urges the valve member towards its first position ismounted between the valve member and retaining means which are adjustable by a screw, so that the compression in the spring can readily be adjusted. ln this manner, it is possible to use a single design of control valve for hydraulic circuits which are required to operate at different pressures, the spring compression being adjusted to suit the valve to the particular operating conditions. I

The present invention will now be more fully described by way of example with reference to the accompanying drawings, in which:

FIGS. l and 2 are ldiagrammatic drawings of aA dockboard installation having a dockboard raising mechanism operated by a hydraulic cylinder,

FIG. 3, is a diagram of a hydraulic circuit in accordance with the invention,

FIGS. 4A, 4B and 4C are sectional views of a pressure operated control valve used in the hydraulic circuit, these drawings showing the valve with its valve member in three different positions, and

FIGS. 5 and 6 are sectional views of a hydraulic cylinder and piston combination used in the hydraulic circuit of the present invention.

Referring firstly to FIGS. 1 and 2, these illustrate diagrammatically the dockboard raising mechanism which is described in detail in my copending U.S. Pat. application Ser. No. 50,'255 filed concurrently herewith. The dockboard is hinged at its rearward edge ll to the base structure of the dockboard installation, and the mechanism for raising the dockboard includes a bellcrank type lever 12 pivotted at its outer end to the dockboard and at its inner end to a tension link 13 by which the lever is connected to the base structure. The lever l2 is movable by means of a hydraulic cylinder and piston combination 14, the inner end of the hydraulic cylinder being pivotally attached to the base structure at a point near to or coincident with the pivotable attachment of the link 13, and the piston rod being pivoted to the lever at a point l5 intermediate its ends. Both the lever l2 and the link 13 are formed of two identical laterally spaced parts. This form of mechanism has advantages over prior mechanisms, in which generally the hydraulic cylinder and piston com bination acts directly between the dockboard and the base installation; however the present invention is only concerned with a hydraulic circuit and not with any particular form of dockboard raising mechanism, and the circuit in accordance with the invention can be used in any dockboard raising mechanism having a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with pressurized fluid. l

The hydraulic circuit of this invention is however of particular value where the dockboard has a lip (such as lip 16 shown in FIGS. l and 2) hinged to the forward edge thereof and movable between a collapsed position and an extended position in which the lip forms an extension of the dockboard, and wherein there is provided a hydraulically operated mechanism which both raises the dockboard and extends the lip. FIG. 1 illus trates the lip raising mechanism, which is described in full in my copending U.S. Pat. application No. 50,098 filed concurrently herewith. The basic components of 'this lip operating mechanism are a link 17 operatively connected to the lip and which serves to raise the lip when moved forwardly, a crank member 18 having one arm pivoted to the link 17 and a second arm pivoted to a rod 18' extending rearwardly, and a bell crank 19 having one arm operatively connected to the rod 18' and the other arm connected by link 19 to the lever l2. The components 17, I8, 18', 19 and 19' are so ar` ranged that as the lever l2 raises the dockboard, the link 19' rotates the bell crank 19 in such manner as to pull the rod 18' rearwardly, this movement causing rotation of the crank member 18 in such manner as to move the link 17 forwardly and thus to raise the lip. The arrangement is such that the lip is raised from a collapsed position to the extended position on movement of the dockboard between an intermediate and a fully raised position.l Since the lip operating means are indirectly operated by the hydraulic cylinder 14, these lip operating means impose forces on the cylinder and piston combination additional to those imposed by the weight of the dockboard while the lip is being moved to its extended position, causing an increase in pressure in the cylinder during this stage of the operation.

The hydraulic circuit which controls the supply of hydraulic fluid to cylinder 14 is shown diagrammatically in Flg 3. The hydraulic circuit includes an electrically operated pump 20 arranged to pump fluid from a reservoir 2l to the inner end of the hydraulic cylinder 14 via a pressure operated control valve 24 interposed between the pump and the cylinder. A strainer 22 is provided at the inlet side of the pump and a relief valve 23 is provided at the outlet side of the pump, this relief valve being arranged to return hydraulic fluid to the reservoir if the pressure at the outlet side of the pump should become excessive. Although the relief valve 23 is shown separate from the control valve 24 in the diagrammatic representation in FIG. 3, nevertheless these two valves may be combined in a single valve body as will be described.

The pump is connected to the control valve 24 by first conduit means 26, and the control valve is connected to the hydraulic cylinder be second conduit means 27. In the embodiment shown the second conduit means is divided into two branches, namely a first branch 27a for outlet flow from the valve which branch is provided with a check valve 29 to prevent reverse flow of fluid back into the valve through this branch, and a second branch 27b which provides a direct connection between the hydraulic cylinder and the control valve. Again, in FIG. 3 the check valve 29 is shown separately from the control valve, but these two valves can be incorporated in the same valve body as will be described. Third conduit means are provided, as in dicated at 28, which provide a return flow from the control valve to the reservoir, so that fluid can flow back from the hydraulic cylinder through the control valve and to the reservoir under conditions which will be described. The conduit means 28 include branch conduit 28a in which the relief valve 23 is situated.

The piston 14a of the hydraulic cylinder is itself provided with further valve means indicated diagrammatically at 3l, and these valve means are operable to allow fluid to pass through the piston when this nears or reaches the end of its stroke. In this situation, the fluid supplied to the inner end of cylinder 14 flows through the piston to the outer end, and' then returns to the reservoir through a conduit 32.

The control valve 24, pressure relief valve 23, and non-return valve 29 are incorporated into a single valve body, which is illustrated in FIGS. 4A, 4B and 4C. Although the valve shown in these drawings performs the functions of the basic control valve, the non-return valve, and the pressure relief valve, for convenience this combined valve will hereinafter be referred to simply as the control valve.

As shown in thev drawings, the control valve 24 has a valve body 35 provided with a central bore having these bore portions while forming a substantially fluid tight seal with the walls thereof. A relief port 40a connects the sides of the larger portion of the valve member, and this port allows for escape of any fluid which may enter behind the large portion of the valve member. The larger bore portion 37 contains a compression spring 41 which is received in a recess in the larger end of the valve member and urges this valve member into a first position which is shown in FIG. 4A. The outer end of spring 4l is held in place by a disc 44 attached to the inner end of a screw 45 which is in threaded engagement with a plug 46 screwed into the outer end of' bore portion 37, the plug 46 also serving to close this outer end of the bore portion 37. The screw 45 is adjustable within the plug 46 to allow adjustment of the compression of the spring 41 thereby adjusting the operating characteristics of the valve.

The valve body 35 has screwed sockets 50, 5l and 52, for attachment to the conduit means. Socket 50 is connected to the pump via the first conduit means 26, socket l is connected to the inner end of cylinder 14 by the secondv conduit means 27, and the socket 52 is connected to the reservoir by the third conduit means 28. Portions of these conduit means are indicated in FIG. 4A.

Socket 50 communicatesv directly with the inlet end of the small bore portion 36, and this small bore portion thus forms a continuation of the first conduit means and the pressure within this conduit means is at all times applied directly to the small end of'the valve member so urging this valve member to move towards the right as shown, compressing the spring 4l.

The second conduit means 27 is divided within the valve body into the branches 27a and 27b. The branch 27a forms a main outlet which leads from a first outlet port 55 in the small bore portion 36 andths branch 27a contains the non-return valve 29 which is constituted by a. ball 29a resting on a valve seat which forms a restriction in this branch 27a. The outlet port 55 is so positioned in the small bore portion as to be obturated by the valve member when this is in its first position as shown in FIG. 4A, so preventing flow of fluid from the first to the second conduit means, but is uncovered by the valve member when this is in its second position as shown in FIG. 4B, so that in the latter position the valve member allows direct communication between the inlet at the end of bore 36 and the main outlet, so that fluid can flow from the pump to the l cylinder via the first and second conduit means.

The branch 27b ofthe second conduit means leads to a second inlet port 56 positioned in the larger diameter bore portion so as to be obturated by the larger portion of the valve member when in its second position, but

uncovered by the valve member when in its first position.

The third conduit means 28 leads directly to an outlet 57 in Vthe side of the large bore portion 37, and this outlet 57 is positioned so as not to be obturated by the valve member in its normal range of movements. Accordingly, when the inlet port 56 is uncovered by the valve member in its first position, there is direct cornmunication between the second and third conduit means via the inlet port 56 and outlet 57.

The third conduit means 28 is also connected to a branch passageway 28a within the valve body, and this branch passageway leads into a further, third, port 58 in the small bore portion 36. This port 58 is displaced axially from the outlet port 55 and is positioned so as to be obturated by the valve member in both its first and second positions, but uncovered by the valve member when this moves to the extreme position shown in FIG. 4C under the influence of excessive pressure in the first conduit means. The port 58 thus acts in conjunction with the valve member to provide a pressure relief valve performing the function of the valve 23 shown in the circuit diagram.

In addition, a further pressure relief valve may also be provided on the outlet side of the pump, since many such pumps include a relief valve as an integral part.

The hydraulic cylinder 14 is shown in more detail in FIGS. 5 and 6. As previously explained the inner end of the hydraulic cylinder is connected to the second conduit means 27, so that pressurized fluid supplied by this conduit means acts against the piston 14a urging this outwardly from the position of FIG. 5v to that of FIG. 6. The outer end of the cylinder is connected to the reservoir by a conduit 32, the outer end of the cylinder being otherwise closed by means of a counter piston S9 which seals around the piston rod and which is held in place by a circlip 59a.

Also in accordance with the invention, valve means are provided as indicated generally at 3l which communicate with the hydraulic cylinder circuit and which are operable to allow flow of fluid from the circuit to relieve pressure when the piston nears or reaches the end of its stroke. These valve. means are mounted within the end of piston rod 60 which extends through the piston 14a and is secured thereto by a nut 61. This end of the piston rod is axially bored to provide an inlet passage 63 leading to a valve seat 64, and an outlet passage 65 of smaller diameter extends inwardly from the valve seat and communicates with a crossbore 66 in the piston rod at the outer side of the piston. A ball valve member 68 is urged onto the valve seat 64 by means of spring 69, which is held in place by pin 70. The ball valve member is arranged to be lifted off the seat 64 by means of a push rod 72 extending within the passage 65, this push rod bearing against a transversely mounted rod 73 which is fitted loosely within the crossbore 66. The ends of rod 73 are held by a collar 75 which loosely surrounds the piston rod.

The valve actuating means also includes two screws 77 extending through the counter piston 59, and positioned to engage the collar 75 as the piston nears the end of its stroke as indicated in FIG. 6. The screws 77 are adjusted so that when the dockboard has been fully raised, and its lip has been fully extended (as described in my copending application No. 075,584 referred to above), the screws contact the collar 75 so that on a slight further movement of the cylinder the valve is operated (as shown in FIG. 6) and allows fluid to pass through the piston from the inner end to the outer end of the cylinder, thereby limiting the fluid pressure up in the cylinder circuit. The fluid which passes through the piston at this stage leaves the outer end of the cylinder via the conduit 32 and returns to the reservoir. The screws 77 can be adjusted in position so as to limit the stoke of the piston quite precisely, and this is a very useful feature of this valve actuating arrangement since it ensures that the dockboard mechanism is fully operated, while preventing any overrun of the piston rod which would cause strain on the mechanism and which would cause an undesirable rise in pressure in the cylinder. f

Operation of the dockboard is commenced with the dockboard in a horizontal position. A switch is operated to startthe pump 20, and as soon as the pump is operating at normal pressure the pressurized fluid passing through the first conduit means 26 acts against the valve member 40 and causes this to move from its first position (FIG. 4A) to its second position (FIG. 4B). ln this second position fluid can flow via outlet port 55 to the second conduit means 27 and thus to the inner end of the hydraulic cylinder, so raising the dockboard. Operation of the pump motor is continued (as by holding the switch closed) until the dockboard is fully raised and the lip fully extended (as described in my copending Pat. application No. 075,584), and at this point the pump motor is stopped.

The lip operating means described in copending Canadian Pat. application No. 075,584 (U.S. Ser. No. 50,098) includes a system of links and levers actuated on movementof the dockboard between intermediate and raised positions to raise the lip from a collapsed position to an extended position in which the lip forms an extension of the dockboard. Since the lip operating means is operated indirectly by the hydraulic cylinder itself, the lip operating means imposes forces on the hydraulic cylinder and piston additional to those imposed by the weight of the dockboard itself. The pressure within the hydraulic cylinder is therefore not constant, and in the final stages of movement of the dockboard upwards, the pressure within the hydraulic cylinder builds up as increased energy is required to operate the lip raising mechanism. The screws 77 of the hydraulic cylinder are adjusted however so that as soon as the lip has been fully extended the relief valve mounted within the piston rod is operated to relieve the pressure within the inner end of the cylinder and upon operation of this valve the cylinder pressure drops by about one third. This arrangement is self-adjusting in that the pressure automatically drops to-a value just sufficient to support the dockboard, any lowering of this pressure resulting in lowering of the dockboard and therefor causing closing of the relief valve. Accordingly, the pressure drop represents the extra pressure required for operating the lip operating means,

. and for overcoming the friction of the parts. This drop in pressure is communicated back to the first conduit means; and the subsequent stopping of the pump motor allows the pressure in the first conduit means to drop sufficiently to allow the valve member 40 to return to its initial position under the influence of spring 4l. The reduction in pressure in the first conduit means when the pump stops is partly due to pressure equalization which occurs between the pump outlet and the cylinder when flow of fluid ceases, and partly to flow of fluid back through the pump. The reduction in pressure is also assisted by operation of the non-return valve 29a, which prevents the pressure in the cylinder being communicated back to the first conduit means so that only a small amount of leakage back through the pump provides a rapid decrease in pressure in the first conduit means.

When the valve member 40 has returned to its first position (FIG. 4A), fluid is free to flow from the hydraulic cylinder via'the second conduit means 27 and branch 27b, through inlet port 56 and into the large diameter bore portion of the valve, leaving through outlet 57 and returning to the reservoir via the third conduit means 28. This flow of fluid allows the dockboard to fall'slowly'onto the platform of a vehicle, and to continue falling to an at rest position after the vehicle has moved away. Accordingly, the simple circuit of this invention allows for semi-automatic operation of the dockboard.

lt will be seen that the provision of a relief valve in accordance with the invention is important not only in preventing overpressure but particularly in reducing pressure within the cylinder when the dockboard has reached its uppermost position. Since any ordinary pressure operated relief valve would have to withstand a pressure in the cylinder circuit high enough to allow the lip operating means to be actuated with certainty, it would have to permit a pressure considerably higher than that required merely for supporting the dockboard. Thus, if a pressure operated relief valve only were to be used, the pressure remaining in the cylinder when the piston reached the end of its stroke would be much higher than that experiencedwith the present invention, and before the control valve member could return to its initial position (FIG. 4 A) considerable pressure reduction would be needed. With the relief valve of this invention, the pressure is instantly reduced to the minimum required for supporting the dockboard.

The non-retum valve 29a is a particularly valuable feature on occasions when the operator releases the pump motor switch just when the lip has become fully extended, but before the relief valve has operated to reduce the cylinder pressure. In this case the cylinder pressure remains high; however the non-return valve isolates the first conduit means from the high pressure so that as soon as a small amount of flow has occurred back through the pump the pressure in the first conduit means is substantially reduced.

The hydraulic arrangement herein described is readily adaptable to different sizes y and designs of dockboard. For example, the adjustability of the valve spring 4l allows the cylinder to operate at a higher or a lower pressure, as desired and also the adjustability of screws 77 allows for accurate adjustment of the stroke of the piston. ln fact, with different sizes of dockboards, it is only necessary to provide different pumps and motors; the cylinder sizes can be the same, and the same valve can be used for the different fluid pressures merely by adjustment of the compression of the spring 41 by screw 45.

Also, the valve of this invention is very simple and cheap to make with standard machine shop equipment.

lclaim:

l. A dockboard installation including a base structure, a dockboard hinged at a rearward edge to said base structure, and a hydraulic arrangement for raising and lowering the dockboard, comprising:

a. a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with pressurized fluid;

b. a pump for supplying pressurized fluid to said cylinder from a reservoir;

c. a pressure operated control valve connected by conduit means to the pump outlet, the cylinder, and the reservoir, said valve having a valve member movable from a first position, in which the valve member allows flow of fluid from the hydraulic cylinder to the reservoir, to a second position in which it prevents flow of fluid from the hydraulic cylinder to the reservoir, said valve member being directly responsive to fluid pressure at the pump outlet so as to move to its second position when the pump is delivering fluid at normal working pressure and biased so as to return to its first position when the pressure at the pump outlet is substantially reduced, whereby the position of the valve member is dependent on the fluid pressure at the pump outlet, so that the dockboard may be raised by operating the pump to supply hydraulic fluid to the cylinder with the valve member in its second position and whereby when the pump is stopped pressure reduction at the pump outlet allows the valve member to Vreturn to its first position, so that fluid may flow from the cylinder to the reservoir allowing the dockboard to lower, the arrangement also comprising a relief valve in the hydraulic cylinder circuit said relief valve being arranged to relieve pressure from lsaid hydraulic cylinder circuit to said reservoir',

and e. valve actuating means arranged to operate said relief valve to relieve pressure to said reservoir as the piston nears the end of its stroke, said relief valve thus operating to reduce pressure at the pump out- 4let at the stage when the pump is stopped thus facilitating the return of the valve member to its first position to allow lowering of the dockboard. 2. A dockboard installation according to claim l, wherein the valve member is urged towards its first position by spring means mounted between the valve member and retaining means adjustable by screw means for altering the compression in the spring means. 3. A dockboard installation according to claim l, wherein the control valve has a bore in which the valve member is slidable, said bore having two aligned portions of small and large diameter respectively, the valve member having portions of corresponding diameters slidable in said bore portions, and wherein the small diameter portion of the bore has its outer end in communication with the pump outlet whereby the smaller end of the valve member is at all times subject to pressure in said pump outlet to move towards its second position, said control valve also including a first port connected to said hydraulic cylinder, said first port being so positioned in the side of saidsmaller diameter bore portion as to be uncovered by the valve member when in its second position, said control valve also having a second port also connected to said hydraulic cylinder, said second port being so positioned in the large diameter bore portion as to be uncovered by the valve member when in its said first position, said valve also having an outlet connected to the reservoir and communicating withl said second port when the valve member is in its first position.

4. A dockboard installation including a base structure, a dockboard hinged at a rearward edge to said base structure, a lip hinged to the forward edge of said dockboard nd movabl between a ol a sed osition to an exten ed position in which the lip fofms as extension of the dockboard, and a hydraulically operated mechanism for raising the dockboard and extending the lip, said hydraulically operated mechanism including a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with hydraulic fluid and said mechanism also including Alip operating means operable to raise the lip from the ,and wherein the said hydraulic cylinder is connected to a hydraulic circuit including:

a. a pump for supplying pressurized fluid to said cylinder from a reservoir;

b. a pressure operated control valve connected by conduit means to the pump outlet, the cylinder, and the reservoir, said valve having a valve member movable from a first position, in which the valve member allows flow of fluid from the hydraulic cylinder to the reservoir, to a second position in which it prevents flow of fluid from the hydraulic cylinder to the reservoir, said valve member being directly responsive to fluid pressure at the pump outlet so as to move to its second position when the pump is delivering fluid at normal working pressure and biased so as to return to its first position when the pressure at the pump outlet is substantially reduced, whereby the position of the valve member is dependent on the fluid pressure at the pump outlet, so that the dockboard may be raised by operating the pump to supply hydraulic fluid to the cylinder with the valve member in its second position and whereby when the pump is stopped pressure reduction at the pump outlet allows the valve member to return to its first position, so that fluid may flow from the cylinder to the reservoir allowing the dockboard to lower, the arrangement also comprising c. a relieve valve in the hydraulic cylinder circuit said relief valve being arranged to relieve pressure from said hydraulic cylinder circuit to said reservoir; and

d. valve actuating means arranged to operate said relief valve to relieve pressure to said reservoir as the piston nears the end of its stroke, said relief valve thus operating to reduce pressure at the pump outlet at the stage when the pump is stopped thus facilitating the return of the valve member to its first position to allow lowering of the dockboard.

5. A dockboard installation according to claim 4,

wherein the valve actuating means is adjustable to allow for adjustment of the stroke of the piston.

i lll 1I 

1. A dockboard installation including a base structure, a dockboard hinged at a rearward edge to said base structure, and a hydraulic arrangement for raising and lowering the dockboard, comprising: a. a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with pressurized fluid; b. a pump for supplying pressurized fluid to said cylinder from a reservoir; c. a pressure operated control valve connected by conduit means to the pump outlet, the cylinder, and the reservoir, said valve having a valve member movable from a first position, in which the valve member allows flow of fluid from the hydraulic cylinder to the reservoir, to a second position in which it prevents flow of fluid from the hydraulic cylinder to the reservoir, said valve member being directly responsive to fluid pressure at the pump outlet so as to move to its second position when the pump is delivering fluid at normal working pressure and biased so as to return to its first position when the pressure at tHe pump outlet is substantially reduced, whereby the position of the valve member is dependent on the fluid pressure at the pump outlet, so that the dockboard may be raised by operating the pump to supply hydraulic fluid to the cylinder with the valve member in its second position and whereby when the pump is stopped pressure reduction at the pump outlet allows the valve member to return to its first position, so that fluid may flow from the cylinder to the reservoir allowing the dockboard to lower, the arrangement also comprising d. a relief valve in the hydraulic cylinder circuit said relief valve being arranged to relieve pressure from said hydraulic cylinder circuit to said reservoir; and e. valve actuating means arranged to operate said relief valve to relieve pressure to said reservoir as the piston nears the end of its stroke, said relief valve thus operating to reduce pressure at the pump outlet at the stage when the pump is stopped thus facilitating the return of the valve member to its first position to allow lowering of the dockboard.
 2. A dockboard installation according to claim 1, wherein the valve member is urged towards its first position by spring means mounted between the valve member and retaining means adjustable by screw means for altering the compression in the spring means.
 3. A dockboard installation according to claim 1, wherein the control valve has a bore in which the valve member is slidable, said bore having two aligned portions of small and large diameter respectively, the valve member having portions of corresponding diameters slidable in said bore portions, and wherein the small diameter portion of the bore has its outer end in communication with the pump outlet whereby the smaller end of the valve member is at all times subject to pressure in said pump outlet to move towards its second position, said control valve also including a first port connected to said hydraulic cylinder, said first port being so positioned in the side of said smaller diameter bore portion as to be uncovered by the valve member when in its second position, said control valve also having a second port also connected to said hydraulic cylinder, said second port being so positioned in the large diameter bore portion as to be uncovered by the valve member when in its said first position, said valve also having an outlet connected to the reservoir and communicating with said second port when the valve member is in its first position.
 4. A dockboard installation including a base structure, a dockboard hinged at a rearward edge to said base structure, a lip hinged to the forward edge of said dockboard and movable between a collapsed position to an extended position in which the lip forms an extension of the dockboard, and a hydraulically operated mechanism for raising the dockboard and extending the lip, said hydraulically operated mechanism including a hydraulic cylinder and piston combination for raising the dockboard when the cylinder is supplied with hydraulic fluid and said mechanism also including lip operating means operable to raise the lip from the collapsed position to the extended position on movement of the dockboard between intermediate and raised positions, said lip operating means imposing forces on said cylinder and piston combination additional to those imposed by the weight of the dockboard while the lip is being moved to its extended position, and wherein the said hydraulic cylinder is connected to a hydraulic circuit including: a. a pump for supplying pressurized fluid to said cylinder from a reservoir; b. a pressure operated control valve connected by conduit means to the pump outlet, the cylinder, and the reservoir, said valve having a valve member movable from a first position, in which the valve member allows flow of fluid from the hydraulic cylinder to the reservoir, to a second position in which it prevents flow of fluid from the hydraulic cylinder to the reservoir, said valve member being directly responsive to fluid pressure aT the pump outlet so as to move to its second position when the pump is delivering fluid at normal working pressure and biased so as to return to its first position when the pressure at the pump outlet is substantially reduced, whereby the position of the valve member is dependent on the fluid pressure at the pump outlet, so that the dockboard may be raised by operating the pump to supply hydraulic fluid to the cylinder with the valve member in its second position and whereby when the pump is stopped pressure reduction at the pump outlet allows the valve member to return to its first position, so that fluid may flow from the cylinder to the reservoir allowing the dockboard to lower, the arrangement also comprising c. a relieve valve in the hydraulic cylinder circuit said relief valve being arranged to relieve pressure from said hydraulic cylinder circuit to said reservoir; and d. valve actuating means arranged to operate said relief valve to relieve pressure to said reservoir as the piston nears the end of its stroke, said relief valve thus operating to reduce pressure at the pump outlet at the stage when the pump is stopped thus facilitating the return of the valve member to its first position to allow lowering of the dockboard.
 5. A dockboard installation according to claim 4, wherein the valve actuating means is adjustable to allow for adjustment of the stroke of the piston. 